Parents are becoming increasingly concerned that their children may be harmed when they are out of their parent's sight. Almost daily the media reports events involving small children being harmed when the small children have wandered from sight of their parents. Furthermore, in today's increasingly mobile society families with small children regularly visit malls, amusement parks and other public places where crowds of people are found which provide an environment where small children can be harmed or become lost or wander from sight of their parents because of their natural inquisitiveness, tendency to explore their surroundings, or their desires to be free from control of their movements by their parents.
Devices are commercially available to limit or monitor movements of children. Devices exist for tethering children to their parents. Further radio systems are commercially available which generate an alarm when children move outside a radius from a radio receiver which receives transmissions from a transmitter worn by children. The tethering devices have a limited restraint radius and create animosity between a child and the parents. The radio systems have a fixed radius of approximately fifteen feet which is too small to permit useful monitoring if a parent does not wish to totally keep a child in sight and cannot be used for tracking.
Numerous radio tracking systems have been proposed which utilize radio communications to locate a mobile radio transmitter and/or to determine when a mobile radio transmitter carried by a person has exceeded a set range measured from a radio receiver. These systems have one or more radio transmitters which broadcast a coded identification of each radio transmitter which is received by a radio receiver and processed to determine the distance and, in some of these systems, the direction between each transmitter and receiver. See U.S. Pat. Nos. 4,785,291, 5115,223, 5,119,072, 5,245,314, 5,289,163, 5,307,053 and 5,357,259, Patent Application WO 87/06748, U.K. Patent Application GB 2182183A and Japanese Patent Application No. 64-311842. A wide range of implementations of radio tracking systems are described in the above-referenced patents and published applications.
The determination if a mobile radio transmitter has moved out of range from a radio receiver receiving an identification code of the radio transmitter is accomplished in many different ways in these patents and applications. Two ways which are described for determining if a mobile transmitter has moved out of range are by determining if the received identification code signal has dropped below a predetermined signal strength or the received identification code signal has not been received for an elapsed time interval.
Radio communication systems which are designed to determine when a mobile transmitter worn by a person has moved outside of a set range and/or to track a person encounter severe problems because of (1) limitations of transmitter power imposed by the Federal Communications Commission which limit broadcast power below 100 milliwatts, and (2) various environmental factors which cause interference, fading, or signal attenuation of the identification code signal which is periodically sent from the mobile radio transmitter to the monitoring radio receiver. The transmitter identification code signal may be severely attenuated by passage through the bodies of people or other structures in the line of site between the radio transmitter and the radio receiver. The presence of people and structures in the line of sight causes substantial attenuation of the transmitted identification code signal which may cause the identification code of the radio transmitter to be periodically or permanently attenuated below the discrimination level of the radio receiver causing a false indication that the mobile radio transmitter has moved out of a set range and an inability to further track the mobile radio transmitter.
Furthermore, natural fading phenomena, such as Rayleigh fading, which is a function of the transmitting frequency and the relative velocity between the mobile radio transmitter and radio receiver are severely aggravated by low speed movement, such as when a child or patient is walking with a transmitter attached to their person to facilitate their tracking. These fading phenomena affect the determination if a set range has been exceeded and a direction determination of the transmitter relative to the receiver. Additionally, other man-made interferences, such as electrical noise and multipath interference caused by buildings, can periodically cause the identification code signal transmitted from the radio transmitter to be attenuated to a level below the discrimination level of the radio receiver tracking the transmitter which also causes a false indication that the radio transmitter is outside a set range and/or the inability to track the direction of the radio transmitter movement relative to the radio receiver with a directional antenna.
Error correction code may be transmitted in a frame of bits encoding the identification code of the radio transmitter. One or more frames encoding the identification code of the transmitter may each contain a set number of error correction code bits which are processed by the radio receiver to correct minor bit errors such as one or two bits which occur within the identification code frame bits. One well known error correction code for accomplishing this function is the BCH code.
The serial processing of the bits of frames which contain error correction code is typically implemented with a series of EXCLUSIVE OR gates. When a number of bit errors in a frame exceeds the error correction capacity of error correction code, the data within the frame is erroneous. The prior art methods of wireless data transmission do not permit the recovery of valid data bits from a frame containing a number of bit errors which exceed the bit error correction capacity of the error code therein which error correction capacity, for most types of error correction codes, is two bits.
The cumulative effects of mis-synchronization of a radio receiver to receive transmissions from radio transmitters, Rayleigh fading, and man-made noise noticeably reduces the reliability of current digital radio receivers to receive error free data. A gap in a data transmission in excess of 1 millisecond may cause a radio receiver to terminate the receiving process. In a situation of tracking a radio transmitter with a radio receiver which receives a periodic digital transmission of the radio transmitter's identification code, termination of the receiving process results in the correct identification of the radio transmitter not being received. As a result, the transmission from a radio transmitter which is, in fact, within a set range of a radio receiver which is monitoring the distance of the radio transmitter from the radio receiver is falsely received as being out of range. This results in an erroneous condition of monitoring the distance of the radio transmitter from the radio receiver and further, may cause a panic situation or otherwise cause the person using the radio receiver to not trust the reliability of the radio tracking system.
An analysis of wireless prior art data transmission protocols in accordance with accepted mathematical relationships for their evaluation reveals that they are poorly suited for data transmissions of more than a few characters in length. The following mathematical relationships are used to analyze fading: ##EQU1##
The threshold ST is the receiver threshold detection level and the median SM is the median field strength level. ##EQU2##
The quantity t is the net probability of a fade divided by the mean rate of fading and equals EQU 1/2rF.sub.o (e.sup.+0.693r.spsp.2 -1) (5)
The fading rate F.sub.o is the natural frequency at which atmospheric radio frequency transmissions periodically fade as a function of the channel frequency F.sub.o and the speed of the radio receiver in miles per hour; the fade length t in seconds is the length of fade; the fade below threshold F.sub.R is the time duration in seconds that a transmission drops below the detection capability of the radio receiver; and the probability of message loss P.sub.(error) is the probability that a message transmission will not be completed as a result of a loss of synchronism between the data transmission and the receiver. See S. O. Rice; Statistical Properties of a Sine Wave Plus Random Noise; Bell System Technical Journal, January, 1948; T. A. Freeburg; An Accurate Simulation of Multipath Fading; Paper;1980; Caples, Massad, Minor; UHF Channel Simulator for Digital Mobile Radio; IEEE VT-29; May 1980; and P. Mabey, D. Ball; Application of CCIR Radio Paging Code No. 1; 35th IEEE V.T. Conf.; May 1985 for a discussion of the above-referenced equations.
U.S. Pat. No. 4,868,885 discloses the rapid measurement of a received signal strength indicator (RSSI signal) generated from reception of a received radio frequency signal which is used in a cellular radio system to control handoff. Samples of the RSSI signal are taken successively in time and compared with the larger of the two samples being stored throughout a desired sampling interval. Sample values exceeding the value obtained from an immediately preceding sample time and a value obtained from an immediately succeeding sample time are stored twice while samples values that are less than an immediately preceding or succeeding sample value are never stored. The resulting average is very close to a true average signal amplitude and is unaffected by Rayleigh fading phenomena but is responsive to rapid changes in received signal amplitude caused by obstacles in the transmission path.
U.S. Pat. No. 5,193,216 detects when a radio receiver of the type which receives data transmissions is out of range. The radio receiver responds to a decreasing slope of a RSSI signal after the receiver fails to receive its coded identification code from the transmitter to signal the out of range condition. The U.S. Pat. No. '216 discloses sampling the received signal strength coincident with the detection of a predetermined characteristic of the signal, such as the sync code, so that the signal for which the received signal strength is measured is indeed the desired signal. If at the time the sync code is to be detected there is no signal which is detected, a predetermined number of the most recently stored RSSI values are read. If the slope of the stored RSSI values indicates that the radio receiver was moving toward an out of range condition before the loss of reception, a display is generated upon loss of reception indicating that the radio receiver is out of range from the radio transmitter.
Disclosure of Invention
The present invention is an improved radio tracking system comprised of a mobile radio frequency receiver and at least one mobile radio frequency transmitter. Each radio frequency transmitter periodically broadcasts a radio frequency carrier which is modulated with an identification code which uniquely identifies the broadcasting radio frequency transmitter which is decoded by the radio frequency receiver. The radio frequency receiver has an adjustable range control which sets a maximum range of movement of each radio frequency transmitter measured from the radio frequency receiver that is permissible without the generation of an alert that a radio frequency transmitter has exceeded the set range. The range setting generates a voltage having a numerical value which is compared to a RSSI signal to determine if the set range has been exceeded. When the radio frequency receiver verifies that an identification code transmitted with a modulated radio frequency carrier is assigned to a radio frequency transmitter which is being tracked or monitored by the radio frequency receiver, the radio frequency receiver generates the RSSI signal which is processed by a processor within the radio frequency receiver to compute an average of successively received RSSI signals from each of the radio frequency transmitters being monitored. The average is compared to the numerical value representing the set range by the processor and the processor alerts the user of the radio frequency receiver when the set range for any receiver is exceeded.
Preferably, each RSSI signal is integrated to remove the effects of electrical noise before averaging. The average of RSSI signals and preferably the average of the integrated RSSI signals generated from transmissions of the radio frequency carriers containing the identification code of each radio frequency transmitter being monitored and tracked are compared to the numerical value representing the set range and an alert is generated by the microprocessor (preferably a digital signal processor) of radio frequency receiver when the comparison reveals that at least one of the at least one radio frequency transmitter is outside the set range.
Preferably, the average of the RSSI signals and the preferred average of the integrated RSSI signals is updated to include newly calculated RSSI signals and preferably, newly calculated integrals of the RSSI signals only when each newly calculated RSSI signal or integral thereof differs from the computed average by less than a function of the average so as to exclude from the computation of the average those RSSI signals or integrals thereof which differ from the average by more than the function. This process discards unreliable and statistically aberrant RSSI signals or integrals thereof which unreliable and statistically aberrant RSSI signals or integrals thereof would interject erroneous data into the range determination process. Phenomena, such as interference from people in the line of sight, Rayleigh fading, multipath interference, etc., can cause substantial magnitude variation of the magnitude of successively received RSSI signals or integrals thereof which falsely would be interpreted as motion of a radio frequency transmitter outside the set range which is not occurring and which would cause an erroneous alert to be generated that a radio frequency receiver has moved outside the range.
Once the radio frequency receiver determines that a radio frequency transmitter has moved outside the set range, the user may switch the antenna configuration from an omnidirectional antenna to a directional antenna by pushing a "find me" switch in the housing of the radio frequency receiver to permit directional tracking by the radio frequency receiver. Also, directional tracking may be performed by pushing the "find me" switch any time the user of the radio frequency receiver desires to monitor the position or motion of each radio frequency transmitter being monitored.
A display of the magnitude of successive RSSI signals and preferably, integrals thereof, which are generated in response to the radio frequency receiver detecting the radio frequency carrier containing the identification code of the radio frequency receiver being tracked, is used to locate a direction from which a maximum signal magnitude of the signal radio frequency carrier is being transmitted by the radio frequency transmitter being tracked. The direction from which the maximum magnitude signal is being received, which is detected by displaying the magnitude of a quantity which is a function of individual RSSI signals generated by the reception of sequential transmissions of the identification code of the radio frequency transmitter being tracked, is the true bearing of the radio frequency transmitter relative to the radio frequency receiver. A preferred function without limitation is the integral or average signal magnitude of the RSSI signal which has the effects of noise removed.
The present invention further permits a user of each radio frequency transmitter to press a "panic" switch to generate an alert which the user of the radio frequency receiver responds to by closing the "find me" switch to cause the control processor to change the antenna configuration of the radio frequency receiver from an omnidirectional antenna used for tracking all of the radio frequency receivers to a directional antenna to permit directional tracking of the user of the radio frequency transmitter which transmitted the alert to the radio frequency receiver. The directional tracking process by the radio frequency receiver of each radio frequency transmitter transmitting an alert is the same as the tracking function described above when a radio frequency transmitter exceeds the set range.
The processor of the radio frequency receiver further utilizes error correction code which is transmitted with the frames of information encoding the identification code of each radio frequency transmitter which is being monitored or tracked to reconstruct valid data from frames which cannot be corrected using the error correction code. In a preferred embodiment of the invention, an IDENTIFICATION FRAME GROUP, which is comprised of a plurality of frames with each frame containing bits of BCH error correction code and bits of many of the frames encoding the identification code of the radio frequency transmitter and one of the frame encoding the status of the user of the radio frequency transmitter, is processed by the radio frequency receiver to determine if at least one erroneous uncorrectable bit is contained in any of the frames. Those frames containing at least one erroneous uncorrectable bit, which cannot be corrected by processing with the error correction code, are further processed to reconstruct valid data in the frame containing the at least one erroneous uncorrectable bit by searching for a bit pattern of the erroneous uncorrectable bits being totally within the bits of the error correction code bit field. When the bits of the error correction code of a frame totally contain the erroneous uncorrectable bits within the frame, the data which is the identification code, status of the user of the radio frequency transmitter or any other information may be recovered. The bit pattern is a number of successive bits having an identical numerical value of either zero or one with the number being at least one greater than a number of bits which may be corrected with the error correction code in the frame which contains the at least one erroneous uncorrectable bit. As a result of reconstruction of frames by recovering valid data from frames containing at least one erroneous uncorrectable bit, a greater number of radio frequency carriers containing the identification code of the radio frequency transmitters being monitored are detected. This enables the processing of a greater number of RSSI signals which enhances the data which is processed to determine the range and direction of the radio frequency transmitters being monitored as described above.
In a preferred embodiment of the invention, the identification code of each of the radio frequency transmitters being monitored is encoded in frames containing error correction code. The bits of the frames modulate a subcarrier and the subcarrier modulating the radio frequency carrier. Analog modulation of the subcarrier or digital modulation of the subcarrier may be used. The analog modulation modulates cycles of the subcarrier with bits encoding the plurality of frames of the identification code and any other information such as the information in the IDENTIFICATION FRAME GROUP. Each cycle of the analog subcarrier is modulated by bits at a plurality of separated angular positions. Digital modulation of the subcarrier modulates a pulse width of the subcarrier. The width of parts of the digital subcarrier are modulated with at least one bit of the frames of the information. This form of subcarrier modulation permits the preferred form of data transmission as formatted into the IDENTIFICATION FRAME GROUP to be rapidly transmitted at a low error rate which enhances battery life.
The processing of the detected individual cycles of the subcarrier by the digital signal processor of the radio frequency receiver includes calculating an integral of at least one selected modulated part of each of the individual cycles, numerically comparing each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical values that the selected part may encode to identify a stored range numerically including the calculated integral and substituting for the at least one selected part of each of the cycles the one of the plurality of numerical values representative of the identified stored range including the calculated integral with each numerical value encoding one bit when the subcarrier is an analog subcarrier and at least one bit when the subcarrier is a digital subcarrier. Furthermore, the processing of the detected individual cycles of the subcarrier by the digital signal processor includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculation of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the compared sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced. The compared sample value is preferably replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which exceeds the compared sample value.
The above-described processes, which are performed by a digital signal processor of the radio frequency receiver for processing the modulated cycles of the subcarrier, ensure that reliable detection of the identification code of each radio frequency transmitter is achieved and reliable data which is a function of the RSSI signal generated during the reception of a valid identification code of one of the radio frequency transmitter being monitored is used to determine the range and direction of a radio frequency transmitter relative to the radio frequency receiver. The reliability of the range detecting function and further the tracking function of each radio frequency transmitter upon the generation of an alert by the radio frequency receiver when a radio frequency transmitter moves out of range or further when a user of the radio frequency transmitter pushes the panic switch is directly influenced by the reliability of the detection process of the identification code of the radio frequency transmitter. The RSSI signals, which are used ultimately to determine if a radio frequency transmitter has moved outside the set range and further to track the direction of a radio frequency transmitter relative to the radio frequency receiver, are qualified by an accurate and high speed detection of the identification code of each radio frequency carrier which is transmitted from each of the radio frequency transmitters being monitored. Therefore, a highly accurate detection process of the identification code of each radio frequency transmitter by the radio frequency receiver insures that the maximum number of qualified RSSI signals are presented for further processing which enhances the accuracy of the determination if the range set by the user of the radio frequency receiver has been exceeded and further, the accuracy of the detection of the direction of the radio frequency transmitter relative to the radio frequency receiver.
An example of a system for determining where at least one radio transmitter is located with respect to a set range measured from a radio frequency receiver in accordance with the invention includes each of the at least one radio frequency transmitter periodically transmitting an identification code from each radio frequency transmitter which uniquely identifies each radio frequency transmitter with a radio frequency carrier modulated with a subcarrier with the subcarrier being modulated with the identification code; and the radio frequency receiver having a processor and in response to receiving each radio frequency carrier, the processor determines if an identification code of one of the at least one radio frequency transmitters contained therein, calculates an integral of a received signal strength indicator of each radio frequency carrier determined to contain an identification code of one of the at least one radio frequency transmitter, the processor for each radio frequency transmitter from which periodic transmissions of identification codes are being received by the radio frequency receiver computes an average of the calculated integrals which is updated to include newly calculated integrals only when each newly calculated integral differs from the computed average of the calculated integrals by less than a function of the average of the calculated integrals so as to exclude from the computation of the average of the calculated integrals newly calculated integrals which differ from the average of the calculated integrals by more than the function, compares the average of the calculated integrals to a value representing the set range and generates an alert when the comparison reveals that at least one radio frequency transmitter is outside the set range. At least one radio frequency transmitter transmits an alert to the radio frequency receiver as part of the radio frequency carrier containing the identification code to cause the radio frequency receiver to signal a user of the radio frequency receiver that a user of the at least one radio frequency transmitter is transmitting a change in status of the user of the at least one radio frequency transmitter. The radio frequency receiver has an omnidirectional antenna, the omnidirectional antenna receives the radio frequency carrier transmitted by each radio frequency transmitter which is used to calculate the average of the calculated integrals; and the radio frequency receiver has a directional antenna, the directional antenna after generation of the alert receives transmissions of the radio frequency carrier containing the identification code identifying the radio frequency transmitter from which the identification codes were transmitted which caused the radio frequency receiver to generate the alert and in response to the radio frequency receiver being moved by a user the processor controls display of a magnitude of the integral of each successive received signal strength indicator generated in response to reception of transmissions of the radio frequency carrier containing the identification code of the radio frequency transmitter from which the identification codes were transmitted which caused the radio frequency receiver to generate the alert to permit a user of the radio frequency receiver to locate the direction, from which a radio frequency carrier containing the identification code of the radio frequency transmitter from which the identification codes were transmitted to generate the alert is received, producing a maximum magnitude of the integral of each successive received signal strength indicator relative to the radio frequency receiver whereby a direction of the radio frequency transmitter which is outside of the set range is determined by the user of the radio frequency receiver relative to the radio frequency receiver. The radio frequency receiver has an omnidirectional antenna, the omnidirectional antenna receives the radio frequency carrier transmitted by one of the at least one radio frequency transmitter which contains the alert; and the radio frequency receiver has the directional antenna, in response to reception of the alert from one of the at least one radio frequency transmitter, the directional antenna receives transmissions of the radio frequency carrier containing the identification code identifying the radio frequency transmitter from which the alert was transmitted and in response to the radio frequency receiver being moved by a user, the processor controls display of a magnitude of the integral of each successive received signal strength indicator generated in response to reception of the radio frequency carrier from the radio frequency transmitter from which the alert was received to permit a user of the radio frequency receiver to locate a direction, from which a radio frequency carrier is received from the radio frequency transmitter transmitting the alert, producing a maximum magnitude of the integral of each successively received signal strength indicator relative to the radio frequency receiver whereby a direction of the radio frequency transmitter which transmitted the alert is determined by the user of the radio frequency receiver relative to the radio frequency receiver.
Each radio frequency transmitter modulates cycles of a subcarrier with bits encoding the identification code of the radio frequency transmitter which transmitted the received radio frequency carrier with each cycle of the subcarrier being modulated with bits at a plurality of separated angular positions or with cycles being modulated with groups of bits encoding the identification code of the radio frequency transmitter which transmitted the received radio frequency carrier by modulating a width of parts of the cycles of the subcarrier with pulse width modulation; and for each radio frequency carrier received from each radio frequency transmitter the processor processes detected individual cycles of the subcarrier to calculate an integral of at least one selected modulated part of each of the individual cycles, numerically compares each of the calculated integrals with a plurality of stored numerical ranges which ranges each represent one of a plurality of possible numerical ranges that the selected part may encode to identify a stored range numerically including the calculated integral, substitutes for the at least one selected part of each of the cycles, the one of the plurality of numerical values representative of the identified stored range, including the calculated integral with each numerical value encoding at least one bit of the identification code of the radio frequency transmitter and decodes the plurality of numerical values to produce the identification code of the radio frequency transmitter. The processing of the detected individual cycles of the subcarrier by the processor includes calculating the integral by taking a plurality of samples of each selected modulated part of each of the individual cycles with each sample having a numerical value and each sample is compared with a range of numerical values representing a valid sample which should be included within the calculations of the integral and when the comparison reveals that the sample value is outside the range of numerical values, the calculated sample value is replaced with a value which is a function of the sample values adjacent the sample value which is replaced. The compared sample value is replaced with a value which is an average of at least one sample value which precedes the compared sample value and at least one sample value which exceeds the compared sample value.
Each radio frequency transmitter modulates the subcarrier with at least one identification frame group, each identification frame group comprising a plurality of frames with at least one of the plurality of frames of the identification frame group containing bits encoding the identification code of each radio frequency transmitter, a plurality of bits of error correction code in each frame, synchronization information for synchronizing a clock of the radio frequency receiver, and a command field for encoding the alert transmitted to the radio frequency receiver from the user of the radio frequency transmitter transmitting the alert. The radio frequency receiver receives the radio frequency carrier, detects the bits of the frames modulated on the subcarrier and stores the detected bits, and the processor processes the stored bits of the frames with the error correction code therein to determine if the frames contain at least one erroneous uncorrectable bit which cannot be corrected with the error correction code therein, processes the bits of any frames which contain at least one erroneous uncorrectable bit to determine if the bits of the at least one frame other than the error correction code bits, are valid bits, and causes storing the valid bits of each frame and processes the stored valid bits to decode at least the identification code of the radio frequency transmitter. The processor processes the plurality of bits of each frame determined to contain at least one erroneous uncorrectable bit to determine if the at least one erroneous uncorrectable bit is contained totally in the bits of the error correction code, and upon determination that the bits of the error correction code of each frame containing at least one erroneous uncorrectable bit totally contain the at least one erroneous uncorrectable bit, causes storing as valid bits each of the bits, other than the error correction code bits, of each frame determined to contain the at least one erroneous uncorrectable bit totally in the bits of the error correction code; and processes the stored valid bits to decode at least the identification code of the radio frequency transmitter. The processor processes the plurality of bits of each frame determined to contain the at least one erroneous uncorrectable bit to determine if the bits of the error correction code of each frame containing at least one erroneous uncorrectable bit do not totally contain the at least one erroneous uncorrectable bit, and upon determination that the bits of the error correction code of each frame containing at least one erroneous uncorrectable bit do not totally contain the at least one erroneous uncorrectable bit, discards the bits of the frame containing the at least one erroneous uncorrectable bit. The processing of the bits of each of the frames which contain at least one erroneous uncorrectable bit to determine if the frames contain only valid bits by the processor includes processing the bits of the error correction code contained in each frame which contains at least one erroneous uncorrectable bit to search for a bit pattern of the erroneous uncorrectable bits totally within the bits of the error correction code; and causes storing of the bits other than the error correction code bits as valid bits when the bit pattern of erroneous uncorrectable bits is detected as being totally within the bits of the error correction code.
A second example of a system for determining where at least one radio frequency transmitter is located with respect to a set range measured from a radio frequency receiver in accordance with the invention includes each of the at least one radio frequency transmitter periodically transmitting an identification code from each radio frequency transmitter which uniquely identifies each radio frequency transmitter with a radio frequency carrier modulated with a subcarrier with a subcarrier being modulated with the identification code; and the radio frequency receiver having a processor and in response to receiving each radio frequency carrier, the processor determines if an identification code of one of the at least one radio frequency transmitters is contained therein, calculates an integral of a received signal strength indicator of each radio frequency carrier determined to contain an identification code of one of the at least one radio frequency transmitter, the processor for each radio frequency transmitter from which periodic transmissions of identification codes are being received by the radio frequency receiver computes an average of the calculated integrals which is updated to include newly calculated integrals, compares the average of the calculated integrals to a numerical value representing the set range and generates an alert when the comparison reveals that at least one of the at least one radio frequency transmitter is outside the set range. The second example includes the dependent features set forth above in the description of the first example.
A third example of a system for determining where at least one radio frequency transmitter is located with respect to a set range measured from a radio frequency receiver in accordance with the invention includes each of the at least one radio frequency transmitters periodically transmitting an identification code from each radio frequency transmitter which uniquely identifies each radio frequency transmitter with a radio frequency carrier modulated with a subcarrier with the subcarrier being modulated with at least one identification frame group containing at least one frame comprising a plurality of identification code bits and a plurality of error correction code bits; and the radio frequency receiver having a processor and in response to receiving each radio frequency carrier, the processor detects the bits of the frames modulated on the subcarrier and causes storing of the detected bits, processes the stored bits of the at least one frame with the error correction code therein to determine if the at least one frame contain at least one erroneous uncorrectable bit which cannot be corrected with error correction code therein, processes the bits of any frame which contains at least one erroneous uncorrectable bit to determine if the bits of the frame, other than the error correction code bits, are valid bits, causes storing of the valid bits of the frame, determines if an identification code of one of the at least one radio frequency transmitters contained in the stored valid bits by processing the stored valid bits, processes each radio frequency carrier determined to contain an identification code of one of the at least one radio frequency transmitter to generate a received signal strength indicator comprising a quantity which is a function of at least one received signal strength indicator and compares the quantity to a numerical value representing the set range and generates an alert when the comparison reveals that the one of the at least one radio frequency transmitters is outside the set range. The third example includes the dependent features set forth above in the description of the first example.
A fourth example of a system for determining where at least one radio frequency transmitter is located with respect to a set range measured from a radio frequency receiver in accordance with the invention includes each of the at least one radio frequency transmitters periodically transmitting an identification code from each radio frequency transmitter which uniquely identifies each radio frequency transmitter with a radio frequency carrier modulated with a subcarrier with a subcarrier being modulated with the identification code; and the radio frequency receiver having a processor and in response to receiving each radio frequency carrier, the processor determines if an identification code of one of the at least one radio frequency transmitter is contained therein, produces a received signal strength indicator of each radio frequency carrier determined to contain an identification code of one of the at least one radio frequency transmitter, the processor for each radio frequency transmitter from which periodic transmissions of identification codes are being received by the radio frequency receiver computes an average of the received signal strength indicators, compares the average of the received signal strength indicators to a numerical value representing the set range and generates an alert when a comparison reveals that at least one of the at least one radio frequency transmitter is outside the set range. The fourth example includes the dependent features set forth above in the description of the first example.
A method for determining where at least one radio frequency transmitter is located with respect to a set range measured from a radio frequency receiver in accordance with the invention includes the foregoing steps performed by the radio frequency receiver as described in the above-referenced examples.